Recently for the purpose of saving fuel consumption of cars in compliance with a social demand for energy saving, compounding of a white filler such as silica or aluminum hydroxide is employed in many cases as a filler ensuring compatibility of a low heat build-up characteristic with a grip performance on a wet road surface.
In a white filler such as silica or aluminum hydroxide, there is a tendency that particles thereof are bonded to each other during kneading and are agglomerated, which results in a problem that a compounding viscosity becomes too high and an extrusion processability and a sheet processability of a kneaded product are deteriorated. In order to improve dispersion of silica in a rubber, it is necessary to make a kneading time longer. Further, since a portion of a surface of a silica particle which is not covered with a silane coupling agent is acidic, there is a demerit such that the silica particle adsorbs a basic substance used as a vulcanization accelerator, and therefore, vulcanization of a rubber composition does not proceed sufficiently and a storage elastic modulus is not increased. Therefore, improvement of processability of a rubber composition comprising a white filler has been demanded.
Methods of improving processability and the like of a rubber composition comprising a white filler such as silica has been reported. For example, Patent Document 1 describes a technique such that an amide compound having a polar terminal showing a weak chemical reactivity to silica and a non-polar terminal showing a weak chemical reactivity to an elastic polymer is added to a rubber composition comprising silica as a processing aid for improving dispersion of silica in the rubber composition. Further, in Patent Documents 2 to 5, a technique of adding fatty acid monoethanolamide or fatty acid diethanolamide to a rubber composition comprising a white filler such as silica.